It's a ticking time bomb the further you step away from mainstream writing software. I've heard too many horror stories. Working in Docs or Word, backing up to the cloud, and having downloaded and external backups would be pretty much foolproof. I've never seen a platform with writing software that was worth abandoning the basics.
Also, with how quickly people throw accusations of A.I around these days, a software with version history might be essential in the next few years. Heck, even if you don't necessarily care about having to prove it one day, those version histories usually persist through a lot of catastrophes.
I'd say at the very least, if an author chooses to use a different writing software, that they should copy-paste it into something like words or docs. I have plenty of notes in different places so that way even if all that could reasonable go wrong goes wrong, it's just be a mild inconvenience to bounce back. Now that I think about it though, I haven't made a plan for solar flares, so of course a physical copy is a must.
I just scanned the other replies, and it seems most people agree on using something like Google Docs. The only downside is their character limit if you're a long-form writer, and they've been getting pushy with A.I proofreading over spellcheck, but it Docs is a good place to keep a master copy, and I find the formatting is preserved pretty well when uploading to various platforms.