When and especially WHY did Märklin begin to caution the use of "kontakt-gleis" ( instead of "schalt-gleis") because the longer closed circuit might damage solenoids in turnouts and signals ?

After all, back before "schalt-gleis" was introduced around 1964/65 the "kontakt-gleis" was heavily used for triggering solenoids in turnouts / signals. And Märklin still uses "kontakt-gleis" to trigger level crossings.

Have the solenoids changed ?

Thank you very much for that qualified post.

Märklin added cut-off switches to the solenoids for C track and K track turnouts to prevent them from overloads. So obviously those solenoids are less robust than earlier models.
With digital operation, effective track voltage can be around 22 V - much higher than the 16 V used with analogue operation.

Hi
For quite some time (before WW2) Märklin was using large single solenoids to drive switches using a rachet mechanism
Here is a 1950 solenoid with ratchet for a signal

.
Then, in 1951, M released full size single contact track and double solenoids motors on switches and signals.
Here is the first appearance of double solenoid motors here on a 4 solenoid DSS (Double Slip Switch)

This is to show that the solenoids are getting smaller so faster but less resistant to overheating

The tolerance for trains to stay on those contact tracks was medium and permanent damage started appearing after one minute (bending of the nylon solenoid core that made harder for the metal core to move.
Then around 1964 /1965 appeared the slider-operated contact tracks (Schaltgleis). Not only those were direction dependent but they would deliver only a short pulse reducing drastically the risk of burning solenoids.
Later in 1996, Märklin introduced its C Track with its compact solenoids with end switches. Very fast, compact but burning rapidly after a few seconds
Cheers
Jean