When is the LB revert gonna be announced?

It won't be?

here is what Riot Meddler said about Leblanc in his quick gameplay Thoughts

After Rengar ships in 8.4 we'll be looking at LeBlanc. Current plan is for a similar level of change to Rengar's, likely including some degree of moving back to things about old LeBlanc that worked well (both for LeBlanc players and for game health). Not certain on timing of when that work would ship, goal would be to get it onto PBE fairly quickly and, as with Rengar, work on it for multiple patches while getting feedback on it.