new distprefix macro is added but per discussion on Matrix, dist is now set {el9,el10} as otherwise it would inherit dist macro for stream, and el9s/el10s

for i in 9 10 ; do cbs taginfo proposed_updates${i}s-packages-main-el${i}s-build|grep rpm.macro; done
  rpm.macro.dist : '.el9'
  rpm.macro.distprefix : '~proposed'
  rpm.macro.vendor : 'CentOS Proposed Updates SIG'
  rpm.macro.dist : '.el10'
  rpm.macro.distprefix : '~proposed'
  rpm.macro.vendor : 'CentOS Proposed Updates SIG'

Feel free to verify and close if working as you expect

As we have a newer buildsys-macros pkg from inherited tags for this one, but it's now solved for all inherited tags (per #1681 ) that shouldn't define dist, do you want us to just undefine it from these build tags, so that it would use distro value (like distcore, and combining with distprefix to define dist ) ?

Waiting for your feedback and closing in some days if no action

Reading it again I guess you'd want now distcore to be set to el9/el10 and using distprefix, right ? I can then unset dist on these two build tags .
Current situation is now :

for i in 9 10 ; do cbs taginfo proposed_updates${i}s-packages-main-el${i}s-build|grep rpm.macro; done
  rpm.macro.distprefix : '~proposed'
  rpm.macro.vendor : 'CentOS Proposed Updates SIG'
  rpm.macro.distprefix : '~proposed'
  rpm.macro.vendor : 'CentOS Proposed Updates SIG'

So that would mean it would expand to ~proposed.el9 and ~proposed.el10 .. let me know if that's what you wanted as result

closing but feel free to reopen if needed