Was the use of silver on 'optical' telescope mirrors more common in the past? Why? In the past, weren't many astronomical telescope mirrors periodically removed and 'resilvered', and wasn't this actually done using silver?

I'm guessing that this is much less common now - new telescopes are so large and complex that routine invasive procedures like that needed for maintenance of a silver coating would not be acceptable, and mirrors in many if not most older telescopes would have been 'updated' to aluminum.

Was silver used in the past because aluminum coating was too technically difficult, or because silver has slightly better reflectivity in the red, or some other reason?

note: I'm only asking about 'optical' telescopes meaning NIR &amp; Vis, not the longer wavelength IR telescopes where IR emissivity is a concern.
Aluminium coating is a relatively recent process - it became available around the 1920s or 1930s. The Hale telescope arrived just in time to take advantage of this new technology.  (It requires a reasonably good vacuum to work, which probably explains why it took a while to come along.)

Before that - around the mid-1800s - various chemical "silvering" processes, usually involving real silver, were used. These weren't ideal as silver tarnished over time - the silver literally oxidised on exposure to air.  (Aluminium also reacts with air, but forms a thin oxide layer which doesn't harm the image and prevents further oxidation.)  Also some of these silvering processes were either toxic or explosive in application...

Before that - the first telescope mirrors were made of metal that was cast and ground to the required curve then highly polished. The material was "Speculum" a copper-tin alloy. They were poor in quality; far less reflective than later mirrors.  This account gives a nice description of casting mirrors for the Parsonstown telescope using over a ton of molten metal. Two mirrors were necessary for the larger telescopes because the mirror in use had to be taken out and re-polished so often...