Recently I realized that the chronology tab for `source` showed a stark incline.

Upon further inspection I figured this is mostly due to source=maxar and source=swisstopo SWISSIMAGE.

Recently? Or do you mean the steeper-than-average incline from ~end 2008 to ~2016 / 2017?

I thought that tagging like that is discouraged. Is it not?

It is. Source should be tagged on the changeset, not on the object.

What do you think: should users be asked to stop doing this

If some still do this, IMHO yes.

and if so, how? I don't want to offend anyone.

How? Politely, and not only stating the rule / recommendation, but also its reason(s). Some will still be offended, but that's as hard to foresee as it is to avoid.

Try to not let yourself be pulled into nasty arguments and stay polite even if others don't.

The goal is not to convince everyone, but to inform those who'd willingly follow such a recommendation (at least if they understand its purpose) but simply didn't know or think about it, yet, or forgot to apply it.

P.S. I realize that this is a bit nitpicky. It is, however, a pet peeve of mine.

That's quite alright. It's a pet peeve of many other experienced mappers here, too, AFAIK. ;-)

Kind regards,
Raphael (das-g)