There's some relevant advice on Meta.SE:
- Should "Welcome to [site]" (and nothing else) comments be flagged?
All the answers to that question are upvoted and advice ranges from
That question doesn't necessarily involve old "welcome" comments, though.
- When should I flag a comment?
When it will be shown to future readers but offers them nothing of value.
By that metric old "welcome" comments should be deleted. The same post says that such comments are "too chatty", and specifically addresses old comments:
- too chatty comments might be polite, friendly, even informative - but they have nothing whatsoever to do with the post!...these are pretty benign - right up until they're being shoved in the face of every reader two years later.
- When should moderators delete comments?
Same advice as for flaggers: when they offer nothing of value to either the author of the post or to future readers.
Personally, I don't see any harm in leaving old "welcome" messages as long as they aren't hiding other, more useful comments. A similar practice is advocated on Meta.SE for "thanks" comments:
You shouldn't feel obligated to flag them when they are not distracting from anything else. When it distracts from other comments (especially if other comments are hidden as a result or take up more than a small fraction of the height of a page), I think they should be flagged.
Answer to "Should a “thank you” comment be flagged?"
I follow that practice on the site I moderate (Science Fiction & Fantasy) when I see such comments during regular browsing. If the comment is flagged, however, my community has decided that such comments should be deleted. This community doesn't seem to have a problem with these comments (judging from the votes and comments on this question), so if that's the case you can decline flags on old "welcome" comments.