Is BS 5979 still a valid certification?

No. BS 5979, the old British standard for alarm receiving centres, has been withdrawn and replaced by BS EN 50518. A provider still advertising BS 5979 certification is quoting a withdrawn standard, which usually means their marketing material has not been reviewed for years.

This matters mainly as a dating signal. It is not evidence of a bad ARC — many centres certificated under BS 5979 transitioned to BS EN 50518 without any change in quality — but it tells you the website has not been maintained, and it is fair to ask what else has not been.

The current question to ask about a monitoring centre is whether it is certificated to BS EN 50518, by which body, and for which category.

The previous website on this domain advertised BS 5979 among its claimed certifications, which is one of several reasons its content was rebuilt from scratch rather than migrated.