How often is the OSM base map updated?

It isn't updated very often because the update costs a lot of time to be fully calculated => the update happens once every few years.

Quote from FlyingTrumpetGuy

if I'm correct and MC pulls POI data from OSM tags, that issue could become less frequent as tagging is updated to reflect the real-world situation.

this is incorrect. POI data can be added by every player and you can choose in your settings (dispatch center → settings) to only use the POIs that you've set yourself instead of the POIs set by all ploayers.

Quote from FlyingTrumpetGuy

I'm assuming MC also pulls a substantial amount of data from OSM tags (the things we use to define characteristics of... pretty much anything- they define buildings as houses, etc)

that isn't true either. MC only uses roads and ways from MC data to generate a mission. any other map data is ignored for mission generation.

Sadly, it seems that they do not update the road data frequently, if ever. My units use old roads that have not existed in about 8 years (funny enough, about the age of Mission Chief...), and do not use new roads that have existed that long. I have confirmed that the actual OSM data is correct and was correct at the times those roads closed/opened, so the issue is MC not importing new data, probably since the game's launch. I opened a ticket with the devs about it, but got a generic response that they'd look into it. That was months ago and I've heard nothing.

Edit: I should also point out that the *visual* OSM map is updated, it's just the pathing that uses old data. So, the result is that units don't use some roads that show on the map, and use old roads that do not show up on the map.