"RichText Format", a very 'lite' type of wordprocessing format (like .CSV is for spreadsheets, except that doesn't even have (much) formatting). ((Ah, ninjaed, while I was spouting the following...))
Personally, the first thing I did when I got my hands on Win11 was to install LibreOffice as the default aternative to the limited MSOffice that was prepackaged (was it Office365, even? such that you're now continually 'renting' it?). Since OpenOffice/LibreOffice has been a thing, I've never used Wordpad any more, but .odt it (or save as .doc, if it's a document going to someone else) if I want a 'proper' word-processor document. Otherwise, native Notepad for plain-text.
About the only thing about 11's native notepad that I like[1], though, is it's 'recovery' option. If it crashes (or, as I also found out, if I leave the machine running overnight and I haven't taken time to close everything manually and restart-to-update before re-opening everything I want to run overnight, now not in danger of being force-rebooted while I sleep), it does at least let me re-open all .txts that were open (and more... see footnote!). I know there's Notepad+, but I am just so used to working with bog-standard Notepad (whether it's with .txt, .pl, .htm or .svg files ... I don't feel the need for context-sensitive formatting, etc, though I do know that three of those could be enhanced by this) since Windows 3.1 (and probably earlier), and it's been a handy constant, until suddenly it seems someone has 'improved' it.
BTW, iI you install LibreOffice, do remember to configure its autosave (or check that it's active, but the version I installed was the first one that hadn't got it), as the first time 11 surprised me with a overnight reboot, I got my .txt files in Notepad 'back', but the LibreOffice documents ('word' docs, spreadsheets) were back at the prior savepoint again! (Since then, the second 'surprise out-of-hours reboot' at least didn't rewind everything. And, since the second SOOHR, I've taken great care to fulfil any noted pending reboot (and re-open what I do actually need) prior to leaving it unattended. (I could also manually change the 'hours' it has decided to suggest are being "least used", but as it doesn't let me set it to "none, none at all, do you know how ecleptic my sleep/wake patterns are???", I decided against that. Still, if you want to set it to "siesta time", rather than "in the early hours", that is always an option if that suits your running schedule.
Sorry, this is (some of!) my combined general impressions of all the little niggles that I have had to get used to on the 'latest' Windows version, just getting them off my chest in one big blast. I'm on top of it all, now (if, suddenly, I find I've got about 100Gb less disk space than I had a few hours ago, I know I'm probably going to get a "updates will install when you next reboot (or in the middle of the night!)" warning, then after I force the reboot-to-update I suddenly regain those 100Gb or so back again... Good job I'm never on a metered data connection, with that machine, I suppose!
[1] Not sure in which post-Vista version it changed, but they've tried to 'improve' it, but made it more unstable and less flexible in its latest version than before, and prone to crashing (with the 'autorecover' of everything you had open only after the second re-open of Notepad, at which point it opens things I had definitely finished with and closed! It also makes for confusion when it re-opens a 'cached' .txt that has been changed since (by other means), giving a very confusing message about what might or might not be the latest opened version, often leading me to Save As a slightly different name so I can review all versions and try to reconcile what I want... Oh, and if you want to have slightly-edited and as-per-last-savs .txt documents side by side, it won't countenance opening both (in tabs or, as I prefered to set it, seperate windows that seem nonetheless to be still different instances of the same running root process), because it thinks you've made a mistake. You have to make a copy of the stored original, and open that alongside the modified-from-save version. Plus if you use ctrl+ and ctrl- to alter the size of one "notepad text window", it changes them all. Then, after ctrl0ing it bacm to default, you'll likely find that scrolling down a different text document, the font is (also) back to default but line-spacing is still as if the font is the larger scale (like "line and a half" inter-line gap), until you tap ctrl+ and ctrl0 to re-reset that notepad window instance. And the flashing text cursor ("|", but <blink>ing) will often fail to refresh after Alt-Tabbing in and out of the notepad instance, leaving multiple static "|"s in there. If you move the text-chrskr back over it, it flashes again, but only because the static | reverse videos the on/off/on | to be an off/on/off one. Quick fix for that is to align the text cursor over the 'phantom' one(s) and add a tab-space (then delete it again, unless a tab at that point is useful to you). Doesn't get rid of the phantom with normal-space (or line-feed, or printable characters), buf a tab does! Really, so many ways in which the 'new' notepad seems to have been rebuilt with added bugs (one would expect that it should work better with the native window-rendering API than non-MS apps, but the evidence is entirely the reverse).