Yes, omoplatas are considered a variant of ashi-gatame or hiza-gatame.1 3 The IJF Referee Commission has confirmed their legality on multiple occasions,1 2 both as a submission, and as a turnover:
Demonstrator: *uke taps, tori points at armlock*
Lascau: Ippon. Ippon.
Demonstrator: [With this movement,] making the kansetsu-waza? [It's ok?]
Lascau: Ippon. No problem. Ashi-gatame. Ashi-gatame, combination with rolling, and then osaekomi or ashi-/hiza-gatame, depending how is the hand.
They are occasionally seen in competition, and are the signature techniques of some high level judoka (e.g. Huizinga, Munkhbat):
1. IJF Refereeing & Coaching Seminar 2016: Day 1 (6:59:55)
2. IJF Refereeing & Coaching Seminar 2018: Day 2 (2:59:20)
3. Kodokan Katame-Waza: Various Techniques and their Names (1994)