The original 2021-era discussion is https://wiki.openstreetmap.org/wiki/Talk:Key:industrial#Feature_vs._landuse and the next section below it, Talk:Key:industrial#parallel_tagging,_man_made=works
It looks like one other user (Dieterdreist) said " Somehow people have started developing and refining the tagging of industrial sites with the intent to describe production features by using landuse tags. Actually there is already a scheme for factories, suitable for feature tagging, rather than land use: Tag:man_made:works and subtags." and Adamant1 took this as an invitation to go merrily deprecating tags, anything with landuse=industrial + industrial=(product_name), “So far I’ve dealt with industrial=factory, industrial=machine shop, industrial=rice mill, industrial=furniture, industrial=steelmaking, industrial=brewery, industrial=food industry, and industrial=food processing. I’m still going through the other ones…”
Therein lies the rub. A steelmaker makes steel, a rice mill makes rice and some of the others follow that pattern, but does a machine_shop make machines? Most likely, no. A machine shop uses machines to make various custom metal or plastic parts in accordance with the customer’s design or blueprint. Those parts could be pretty much anything, and will change from one client or one job to the next.
So any attempt to deprecate industrial=machine_shop in favour of man_made=works + product=… is going to cause a problem, as “machine shop” is a service while the man_made works are products.
There is a discussion open at Talk:Tag:product=machinery - OpenStreetMap Wiki and the user who originally deprecated industrial=machine_shop is making bizarre suggestions that it be replaced with anything from leisure=hackerspace to craft=*. None of these suggestions make any sense.
This user has a pattern of “deprecating” tags and trying to get them replaced by different tags which they believe to be equivalent, but which are actually different in some way. For instance, a few days ago (on 24-25 Jan) they were edit-warring to blank Tag:workshop=repair_cafe - OpenStreetMap Wiki and replace it with a {{deprecated}} tag pointing to leisure=hackerspace which, again, doesn’t make sense. A hacker space or maker space can be used to create new things, while a repair café only fixes existing, broken things. Not the same beast.
As far as I can tell, the only reason landuse=industrial + industrial=machine_shop was marked as deprecated is that one user doesn’t like that “industrial” is a land use tag. Too bad. We do need a tag for “machine shop” so, if it isn’t in its original “industrial” key spot, then it should go to workshop=machine_shop (or the equivalent positions in ‘shop’ or ‘amenity’ as less-desirable alternates). A machine shop is a “shop” as in workshop, not a “shop” as in a retail store selling machines or something crazy.
I think that the tag should have been left well enough alone and the “deprecated” removed from industrial=machine_shop. My second choice would be workshop=machine_shop, but it seems like a rather pointless change if the old tag is still in use on multiple individual machine shops worldwide.