The two weekday early evening journeys on the 21 that no longer serve Brentwood have now been reinstated, with a catch: they are subsidised by the county council and are operated by EOS. Townlink still run the commercial daytime service and it looks like EOS do not accept Townlink tickets, so if you're not a pensioner you'll have to either pay again or use an Essex Saver ticket, which is valid on both.
The reinstated journeys are numbered 21A and you can see the timetable here.
The main 21 timetable is here.
