I'll never switch from the on-site text box. I've lost posts once or twice, but I will admit generally speaking, once I can shove my brain along to get through a paragraph, I can usually shove my brain along to get through the rest of it in the same sitting, so usually what I lose is like, a sentence that's been sitting there stalled for a month. I feel like if I tried writing my RP posts in a word doc, I'd slip into CS, present, or solo project mode, and that simply doesn't work for me for writing RP posts. I keep having to scroll back up to see what they just wrote (bc I don't want to miss anything but my brain holds memories as well as hands hold water), and after all this time, it just feels off to put them in a word doc, when word docs are meant to last for me. Like I put all my worldbuilding stuff in that kind of thing, yaknow? I don't want pages and pages of posts some for RPs that never got anywhere floating around in word docs all over my fun writing folder.
If I know I'm going to need time or won't have internet connection to send posts or am writing for a discord RP and run into the word limit, I will put them in a little notepad, though. I don't know why it gives me an extra feeling of security, when notepads only started autosaving on shutdowns like... in the last year, but it does, lol, and I've actually never lost a post that way (I've lost OTHER things that way, but not posts). Notepads aren't really meant for permanent status, so I just transfer the post over as soon as it's ready and delete it, haha.