I think the Google Apps for a work aka gmail for your own domain has recently put a limit that "free" legacy accounts can't buy storage. The regular @gmail, and paid domain gmail users can still buy any storage plan.
According to the email reminders that Google keeps sending me, they're flicking that switch in 7 days and moving everyone to "Google Workspace Business Starter" or oblivion. I thought it was called G Suite, I can't keep up.
Not everybody, I think only if one has themselves not opted for any new option (free non business use OR paid business use). I have 5-6 personal domains, for family, maybe a user or two on each. I have opted rightfully non business use, and I am not getting any reminders.