This MSC proposes creating room version 5, which will enforce the signing key
valid_until_ts timestamps proposed in
MSC2076.
The new room version is called 5. The only difference between v5 and v4 is
that v5 rooms enforce the valid_until_ts timestamp on signing keys as
proposed in MSC2076.
It is not yet proposed to change the default room version to v5. Version 5 will be considered a "stable" version.
See also MSC2002, which proposed room v4 but also mentioned that a v5 was anticipated and gave some context for this change.