The FCC database does not appear to have much info on any stations' HD digital broadcasts, so it looks like this kind of action is mostly exploiting loopholes in rules set during the analog era.
I wouldn't call it a loophole anymore. It started out that way, but the FCC has since clarified that relaying a digital subchannel on an analog translator is legal so long as it follows the standard relay rules for translators. If the FCC has acknowledged it's permitted, there's no gray area or loophole any longer.
As far as they know K298BY is a repeater for KANW 89.1 without having to state specifically what part of the signal is being relayed.
I've seen a few FCC filings that specifically mention using an HD subchannel, but, to the best of my knowledge, specifying which channel is not required.
KABG now claims to have an HD broadcast, but no notification filed yet. Is that still required today?
Not sure if it's still required or not, but the FCC website has a reputation for being out of date on updating things like that.
Similar loopholes might also apply to television. KRQE is affiliated with CBS which is listed as the network affiliation on the FCC database, but it also has Fox on 13.2 after it was moved from its' longtime (but not original) home at KASA channel 2 in 2017 since the four major network affiliates supposedly must have different owners in a market. Two major networks on a single station may bypass that rule since it wasn't possible in the analog days.
While one could maybe argue moving a network affiliation to a digital subchannel violates the spirit of the FCC's prohibition against common ownership of the top four stations in a market, the FCC has always allowed an operator to have multiple big four affiliations in a market. I can remember almost 30 years ago in college when KODE 12 in Joplin, MO had a secondary Fox affiliation. It would tell the viewers to set their VCR's for various times in the overnight hours to watch Fox programming. After Fox got the NFC package of NFL rights from CBS, it did the opposite on Sundays during football season and suggested setting your VCR for ABC programming. KTEN in the Sherman/Denison, TX market used to have ABC, NBC, and Fox programming at various times of the day, too. I follow TV less than I do radio. So, this may have changed, but, last I'd heard, digital subchannels didn't have must-carry rights. So, moving Fox from KASA 2 to KRQE 13.2 is a theoretical disadvantage. Getting KRQE 13.2 on cable and satellite could, in theory, provide more of a bargaining chip to the provider and result in reduced compensation to carry the main channel of KRQE. That might not, however, happen in practice.