A lot of this is irrelevant because the OP clearly indicated that battery
was in the trunk of the car!

More often a car battery is mounted adjacent to the engine and usually the
reason for boosting (jump starting) is that it has run down/hasn't been
charged properly, cold weather etc.

In this colder climate of Eastern Canada carrying a set of cables and/or
stopping to 'give somone a boost' is not that unusual.

Battery explosions are rare; but agree a lot of people do not understand the
hazard of making a spark at/near the battery; either when making the initial
boosting connection or after the boosted vehicle has started; due to the
production of hydrogen and oxygen as the battery is being rapidly recharged!
(BTW nowadays most/all North American sold cars now seem to be 12 volt
negative ground).

So, unfortunately the instruction not to connect to the negative
post/terminal of the battery itself is often ignored.

Better understood, but also often ignored, considering that replacing a
burnt out alternator with a rebuilt can typically cost $100 (40 to 50 quid)
installed, is to not run the engine of the boosting vehicle (merely use its
battery to start the stalled one),

What does seem to be better understood is to disconnect the cables from the
boosting vehicle first and also to connect the negative cable at the
boosting vehicle to something negative such as a lift hook on the engine
block etc. Thus any spark at disconnection is away from the battery vents.

I have seen a portion of a battery blow up, forunately no one was injured by
flying acid but the cover of two end cells of the battery were blown right
off! However there was enough 'oomph' and connection left in the battery for
the owner to drive it to the nearby town to buy another battery! I guess,
while doing that, fortunately the battery didn't go 'open' in which case the
voltage might have gone out of sight and blown other car electrics. I think
that was still back in pre-alternator days. You could tell back then that a
vehicle had a weak battery; when the engine slowed down to an idle the
lights would dim. Driving at speed they could be superbright!