Do dormant FF accounts get deleted?
 

A lot of airlines have expiration policies in place for miles.

But do any of them delete dormant accounts?

Yes, they do - my wife had an old AS account from the 90s that had gone dormant and when we tried to reactivate it, they said we could not and should just create a new one.

There is no single answer for all ffp's
Will vary between airline frequent flyer programs.

As u2g0d said, yes they do.

Just to give a data point where this mattered, I had used up my US Airways miles, and then the few miles left had expired. But when the two airlines merged, I couldn't get my US Air miles added to my American lifetime miles.

It might also depend on the exact definition of "dormant" at some airlines. At some airlines logging in "occasionally enough" might be enough (I certainly kept an Asia Miles account alive for over a decade doing nothing more than that, I never had miles in it, I just used to use it for oneWorld alliance availability checking), while for other airlines it might require more than just logging in, I don't know..

I've had both AS and WN accounts purged from lack of activity.

The same thing happened to me - I even had an old statement with the US Airways number, my password, etc. I lost those lifetime miles in the merger (which probably weren't many to be fair).

Yes. SQ will delete any Krisflyer accounts with no qualify activity in 36 months.

Depends is I think the answer. I have a Qantas account from the 1990s which lay dormant for many years. When I started flying locally within Aus again, I added it to my booking and all was as smooth as if I had never been away.
Many years, I think, more than 5 at least...