I recently switched my SSH servers to only listen to IPv6. They're still on port 22, and still pointed to by DNS, yet, while they received bot connection attempts every few seconds when listening for IPv4, they haven't received a single bot connection attempt in the last week since I made the switch.
Though do take note of RFC 7707, "Network Reconnaissance in IPv6 Networks":
IPv6 offers a much larger address space than that of its IPv4
counterpart. An IPv6 subnet of size /64 can (in theory) accommodate
approximately 1.844 * 10^19 hosts, thus resulting in a much lower
host density (#hosts/#addresses) than is typical in IPv4 networks,
where a site typically has 65,000 or fewer unique addresses. As a
result, it is widely assumed that it would take a tremendous effort
to perform address-scanning attacks against IPv6 networks; therefore,
IPv6 address-scanning attacks have been considered unfeasible. This
document formally obsoletes RFC 5157, which first discussed this
assumption, by providing further analysis on how traditional address-
scanning techniques apply to IPv6 networks and exploring some
additional techniques that can be employed for IPv6 network
reconnaissance.
Probably saves them time. You are not a soft target. What incentive do they have to add support when it only gains them access to more hardened systems that most likely are not vulnerable to their next phase.
Bots apparently simply don't bother with IPv6.