[dkim-ops] dkim key "x=" expiration tags -- not in wide use, or need add

Per allowance by RFC

    http://dkim.org/specs/rfc4871-dkimbase.html#dkim-sig-hdr 3.5

        The DKIM-Signature Header Field

            ...
            x=
                Signature Expiration (plain-text unsigned decimal integer;
RECOMMENDED, default is no expiration). The format is the same as in the
"t=" tag, represented as an absolute date
                ...
                    ABNF:

                        sig-x-tag    = %x78 [FWS] "=" [FWS] 1*12DIGIT
            ...

I'm specifying my DKIM keys' intended, target expiration date with "x=" in
the DNS TXT record,
​ e.g.,​


    selector._domainkey.example.com. 5 IN TXT (
      "v=DKIM1; h=sha256; k=rsa; s=email; t=s; x=1525737600;"
      "p=M...B;"
    )

Testing my published records, all PASS -- no errors.

I understand that "x=" MAY be ignored by verifiers,

    http://dkim.org/specs/rfc4871-dkimbase.html

        6.1.1 Validate the Signature Header Field
            ...
            Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (signature expired) if it contains an "x=" tag and the signature
has expired.
            ...

Looking at received/analyzed headers at a collection of mail recipient
servers, including a couple 'robust' mail providers,

    gmail.com
    fastmail.com

The message 'signing timestamp', "t=##########", *IS* present.

But, I do NOT see any evidence of "x=".

Testing with an "x=" <= 'signing timestamp', the messages are NOT rejected
by either of the recipient servers, so it appears they're not *USING*,
rather than simply not *REPORTING*, the "x=" tag for validity checks.

Are "x=" tags simply not in wide use for validity checks?

Or, is there additional config required in the DKIM record spec to
enaure/force its usage?

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