Stefan Nagy
Yes, from all I know source:maxspeed=zone should be sufficient… But this source-tag is really a mess, I didn't read all the arguments on the mailing lists… For example it makes a lot of sense to me what westnordost wrote in a StreetComplete-issue (see https://github.com/westnordost/StreetComplete/issues/492) and I tend to agree that we should use maxspeed:type instead of source:maxspeed. But I didn't take the time yet to make up my mind, so I decided to follow the herd some time ago. And the herd likes source:maxspeed=AT:zone30 and source:maxspeed=AT:zone:30 (see https://wiki.openstreetmap.org/wiki/DE:Key:source:maxspeed and https://taginfo.openstreetmap.org/keys/source%3Amaxspeed#values). I just guess someone had a good reason to propose redundant values…