Electronic d/t timers have been in use for about 25 years. The first electronic timers were almost simultaneous developments by Ken Bauer and Thomas Koster in 1977. The detail in the following bullets outlines the history of timer development class by class. It makes no attempt to cover all available timers: for that, see Current timers in the following section.
In my opinion electronic timers have not got such a clear advantage over mechanical timers for the power, rubber and electric classes because their only benefits are accuracy and ease of setting. That is, unless you use the RCDT facility many of the current timers offer. These are invaluable for trimming on tiny fields.
Despite this I think electronic timers are the inevitable future for all models apart from those light enough to use Tomy Toy or silly putty mechanisms and for those model fliers who live in countries where fuses can still be used safely. Seelig timers disappeared when the clockwork music box parts they depended on were driven off the market by electronic replacements and, as hand grenades too move to solid state fuses, the mechanical Eastern European timers are likely to suffer a similar fate.