Recently I stumbled over the key "lines". According to taginfo there exist about 6500 objects with this key, a number that has been decreasing the last days as I deleted quite a lot of them (the keys, not the objects). From what I see there are 3 different usages of this key:
to tag which public transport lines are serviced here, so this may be a lines=U1 on a railway=subway or even on a railway=platform. Whereever I found that and the object was already part of a proper type=route route=(some public transport thing) for all of the lines keys, I simply deleted them. Given the relatively small number of these object noone can create any reasonable public transport information from these tags. public_transport relations do a far better job for them, and they are much more popular. Given the age of almost every instance of these keys on the objects they clearly predate the public_transport relations, and simply had not been removed when the relations were introduced. So whenever you care for public transport tagging anywhere please check if there are lines keys in the objects you edit, and if their information is contained in a proper relation, then just remove the lines keys.
to tag something that has a different key. I clicked a bit around in the OverPass queries linked from the taginfo page and any instance I found highway=* lines=* it should have been "lanes" instead. There are also some instances of railway=* lines=*, where I guess tracks would be the right key (at least for lines=1 and lines=2). In case you want to touch any of them: prefer to tag every track as it's own way and not use the "tracks" key. I assume that almost any combination of lines with either highway or railway is wrong and should be fixed.
to tag something entirely different. There is one instance of lines=1 in North America where this gives the number of power lines between poles. There are some instances of lines=phone where this tells you that there are telephone wires. There are some instances of lines=no or yes which tell you that this is a basketball court that may or may not have markings. I personally would not touch these taggings as it is clear to a human what they mean, and deleting them without replacing it with a proper tagging would destroy information (how relevant or not they may be).
From my point of view it would be great if all instances of #1 and #2 would be removed from the database. But simply deleting them is not the way forward, one should do it with care. The way to get rid of the greatest numbers of them is to introduce proper public_transport relations where they are missing, e.g. for some of the S-Bahn lines in Hamburg or U-Bahn in Stuttgart. In case you are bored and any of these tags are in an area you care about: check what is the proper tagging, and let's get rid of lines.