In terms of Ubuntu Budgie (and all official flavours) - ubuntu pro is still applicable. We all share the same archive and all security fixes to the underlying software such as graphics, kernel etc applies. So if you intend to run any flavour for more than 3 years you still a similar security assurance as ubuntu itself.
Still - if you are the type of user that enjoys living on the edge - upgrading every 6 months - ubuntu pro obviously has very little interest to you.
certainly won’t mess up any official flavour - well at least not intentionally!
I can’t comment on what will happen with unofficial flavours since they do some weird and wacky things with packages.
I am aware of quite a few business who depend on Ubuntu Budgie. Some of them have graciously donated to us in the past. So yes - ubuntu pro is more applicable to any user / business who have an interest in stability rather than “cutting edge”
The question is how long third party packages would be maintained on a legacy version of ubuntu.
I discussed it with a member of ubuntu discourse. What I got out of it was that there is no guarantee that third party software will be updated security wise to the standard that it would on a current version of ubuntu.
Based on that I would think twice about not upgrading to the next LTS. Not much point to pro if you want to use third party services…
If you depend on third party software then yes you should carefully consider potential security issues - especially browsers.
Personally I would dare to suggest for high risk software such as browsers I would use self contained packages such as flatpak’s and snaps which are portable and dont depend on the underlying distro to any great extent. Deb based third-party software will likely not be maintained beyond the scheduled end-date of 5 years. Sometimes shorter - 3 years or less.