I started taking screenshots of every comment I post and store them in a folder labeled 'Backup'. Sometimes I copy the text into a note app too, which has saved me when posts vanished. Platforms change a lot, so having your own record is CRUCIAL. Make it a habit right after you hit submit.
Oh man, my friend lost a huge recipe thread she'd built for years when a forum just shut down. She was crushed. Now I totally get what @willow760 is saying about making your own copy right away. It takes two seconds and saves so much headache.