I don't see any "-- " in your emails (RFC3676 section 4.3) so how would any application be able to know where email content ends and where a signature starts?

Reply from mobile without double hyphen above signature.

Quim Gil likes to test mobile signatures

Reply after adding double hyphens above signature in Android's gmail.

Quim Gil likes to test mobile signatures

Source of the comment above:

Reply after adding double hyphens above signature in Android's gmail.

--
Quim Gil likes to test mobile signatures

Hyphen hyphen space?

I just found https://github.com/phacility/phabricator/blob/3cf9a5820fc9887f1454decc47c234fedb134c6a/src/applications/metamta/parser/PhabricatorMetaMTAEmailBodyParser.php#L99 ; these are the signatures that are stripped:

$body = preg_replace(
  '/^-- +$.*/sm',
  '',
  $body);

// Mailbox seems to make an attempt to comply with the "standard" but
// omits the leading newline and uses an em dash?
$body = preg_replace(
  "/\s*\xE2\x80\x94 \nSent from Mailbox\s*\z/su",
  '',
  $body);

// HTC Mail application (mobile)
$body = preg_replace(
  '/^\s*^Sent from my HTC smartphone.*/sm',
  '',
  $body);

// Apple iPhone
$body = preg_replace(
  '/^\s*^Sent from my iPhone\s*$.*/sm',
  '',
  $body);

Would it be possible to copy paste our email signature into our phabricator profile to add it to a blacklist?

In T866#16716, @Jaredzimmerman-WMF wrote:

Would it be possible to copy paste our email signature into our phabricator profile to add it to a blacklist?

Currently not, and I would recommend to just use defacto standards like writing "-- " in a separate line right before the signature starts.