Looking at OpenStreetMap's Transport layer, I can see public transit routes, but not any ticket zones (for example Berlin's ABC zones or London's 1-9 Underground zones).
Does this information exist on OSM, and if not are there any proposals for specific tags to add it?
No. This information does not exist on OSM.
There was a discussion about that here with the following arguments:
public transport zones are not geodata. So they should not be mapped within OSM
they are similar to public transport schedules; [...] So any application needing to use them can always pick up the latest from the source.
But as of now, there is no such attribute for stations. There is nothing like zones in the proposals either.
Ticket zone data exists where people who thought it was worth having have imported it and no-one has subsequently removed it. An example is Mansfield's "pay scale area" https://www.openstreetmap.org/way/38387740/history which dates from a partial UK import of NaPTAN data about 8 years ago. However, in the case of data that was imported from an external source and hasn't been kept up to date you're definitely better off using the external source directly rather than trying to second-guess whether OSM is correct or not. For example, I deleted the adjacent pay scale area to Mansfield because I knew it was wrong (it matched neither the current pay scale area nor plusbus zone, both of which had changed since the data had been imported).
So to summarise, you might find the data that you're looking for in OSM, but if you do you'll need to make sure it's regularly updated, and even then you're probably going directly to the data source.
