I think that the translation that's been in use is quite alright, however I believe that the word 'экстренное' still primarily denotes 'urgent' or 'emergency' announcement, like in the case of a storm or other physical, immediate kind of danger. The key word here is rather 'public', because in the context of Telegram it's important to distinguish which message is sent on behalf of Telegram team and which message is more appropriate to inform users about public measures which are external to Telegram, but which Telegram can support in the public interest, like if it were to inform users of an opportunity to get vaccinated in a certain geographical area.

We can't translate ''public service announcement'' as ''экстренное оповещение'', here I suggested more appropriate option for this phrase.