Not only because it is a proposed bridging solution between IPv4 and IPv6, the "version number" of IP is actually somewhat more like an identifier for subprotocols. There is the list [1] of all taken version numbers managed by IANA.

IPv4 is the version 4 because former versions of protocols (0 thru 3) are long obsolete; IPv6 is the version 6 because the version 5 was taken by another experimental protocol; and IPv10 ought to be the version 10 because the versions 6 thru 9 were used for proposed successors to IPv4 (and eventually the protocol of version 6 was chosen). I'm not very sure if other assignments were possible in the current standard process though (e.g. up to IPv14 would be available).

[1] https://www.iana.org/assignments/version-numbers/version-num...

Yes:

"IPv10 allows hosts from two IP versions (IPv4 and IPv6) to be able to communicate, and this can be accomplished by having an IPv10 packet containing a mixture of IPv4 and IPv6 addresses in the same IP packet header."

>From here the name of IPv10 arises, as the IP packet can contain (IPv6 + IPv4 /IPv4 + IPv6) addresses in the same layer 3 packet header.

Winamp did it first