On one hand this is not a big deal, on the other one this is a fallback option on a slow connection and no access to IMAP apps. Someone’s day will be ruined.
And on mobile if you want to edit settings like filters, impossible to do without the html version
Ultimately, I ported my domain to Fastmail, where it’s possible to edit filters in the mobile app. Fastmail also makes it really easy to choose the username from which to reply. You don’t have to create a dumbass alias address.
I would hazard a guess that the number of real and important situations where a critical and irreplaceable legacy system requires access to *Gmail* is close to zero. These systems are unlikely to have access to the internet, and if you’re connecting an *unsupported browser* to a legacy webmail solution, you’re doing things very, very wrong.
What are you thinking of self-hosting? The email server? There are plenty of alternatives to GMail. An email webclient? Just use a non-web client, there are plenty of those too :-).
For a webmail client probably https://squirrelmail.org. It's over 2 decades old, still maintained, and kept to its original simplicity.