The update to Wear OS One UI 4.5 was installed on my Galaxy Watch 5 pro on the 11th of September. I am very sure that after that update I was (still) able to select barometer as a complication. So I do not think that this update to Wear OS One UI 4.5 is related to this ‘glitch’.
EDIT: this is really odd. I changed the language on my phone (Galaxy S21 FE) from Dutch (NL) to ENG (US) and a lot more complications appeared as available (including the barometer). After having switched back to Dutch (NL) I can still select all the complications I could in ENG. So, for me this issue is resolved. However, it seems to be a bug. Where can I report this bug?