How to fireproof steel beam in existing (45 year old) house
Adjacent houses had one of the nastier asbestos boards in the groove seen between the joists and under the metal. Then the regular plasterboard layer. Fireboard is what we put up in the same position, then plasterboard. &quot;Grandfathered in&quot; being the position re regulations.
Legally, we need fireproofing to regulations for steel beams as:

in the case of a small fire, the dwelling should no collapse beyond what would be expected. This is to protect people (say firemen/women) who may be there at the time, or in the aftermath. The fire protection for beams allow them to survive longer without collapsing and perhaps benefit from a water via a fire hose (say) 30 mins after the fire started.

Your insurance claim could be invalidated if there was no fire protection for beams, or the fire protection was inadequate given regulations.  This reinforces the home owners willingness to attend to #1 above.