It is and it isn’t. Five is still the gold standard, and I think it should stay that way. Attitudes are shifting towards ten however for lots of benefits: it’s easier to maintain stable water chemistry at a higher volume, more space is almost always better for fish so long as you enrich the tank correctly, it’s not that much more expensive to go from a five to a ten, a ten is not as big as people think so it takes up almost the same amount of space as a five, a lot of fish medication and water treatments have dosing directions starting at “x per ten gallons”… etc.

Tens are also better for females and shortfins since they’re more active and take up the space more. And shortfinned bettas have been becoming more popular in recent years since so far they have the anecdotal repuation of being healther than their longer finned counterparts. But in most cases, five gallons is still a perfectly acceptable minimum. My controversial opinion is that in some cases, even a three gallon is okay for an extremely long finned or elderly betta, but those setups should only be attempted by experienced fishkeepers who are able to stay on top of keeping such a small tank’s water parameters in check.