Onioncat is still actively developed. The shift to HSv3 has implications that break automatic and secure routing since addresses have 260 bit routing while IPv6 has only 128 the full space can never be used since it would take over the space reserved for the public internet. HSv2 works all right for this right now, but when Tor deprecates it there won’t be any seamless replacements left.
Bernhard proposes a HSv2-3 compatibility system to allow for HSv3 lookups:
https://lists.torproject.org/pipermail/tor-talk/2019-September/045302.html