Officially the stopover on a 1-way was never allowed; however, in practice, it ALWAYS was allowed. Even domestically. At least in my many data points. This was a big win if you booked in F. Practically this unofficial policy was probably permitted because the 4-hour connection window is onerous.

That said, if you booked a round trip, you were granted one free stopover, and all additional stopovers were $100 extra. This meant that even if the above was disallowed "officially", it was quite easy to get 4 separate segments officially by committing to a round trip. Today, I tried to a 3-segment roundtrip: outbound, with a <4 hour connection; and return, with a stopover. The system rang up with the stopover price of $125**. I balked because I should get one free stopover. The agent asked about it and was told no stopovers in US at all. Not free, not for $100 extra (or $125**). When I called back it was same agent. Either this was a legit policy change, or it just necessitates more HUCA another time. What concerns me is even if I get an agent that will do it, I'll probably have to pay $250 for those extra legs.

** $125 always seems to ring up (+taxes) for the stopover. I've insisted to have the $25 phone booking fee waived, but they claim it's not the phone fee. Shrug, it seems like phone fee, but maybe the stopover raised in price.

The biggest trouble in the past was agents would routinely ring it up as 2-segments and it priced that way. Now, the troubles may be larger. Anyway, it was always suspect to me whether this is a great deal or not. For me, it was nice to take some short flights for approx 1cpp. That made the deal look like 12.5k SQ + $125 vs 10k + 10k on United. Now, it appears to be 25k SQ + $250 -vs- 50k UA ***. I guess it's worth it in both cases, but mostly to manage smaller point balances. The opportunity cost with variability in phone agent probably kills it for me.

*** UA seems to have eliminated short flights for 10k. It's variably priced, but usually 12.5k

Haven't found a SQ source, but a blogger claims none allowed domestically. It appears to exclude Canada from that domestic rule. I think this was tacked on a devaluation somewhere. This may be a YMMV now. My last successful stopover segments was in August 2018 without issue