Tech Level slider is a feature that was present in Tiberian Sun and allowed for modification of max tech level that's available ingame. In Red Alert 2 the only thing that allows to disable higher tech level weapons is Meat Grinder.. which is a very limited method of reducing TL since it only disables a predefined set of weapons.
Tech Level slider allows for such modifications in any game mode and can be used for various game settings (T1 game, T2 game, T3 game etc.). Modders could also use Tech Level slider as an alternative of "Superweapon toggle" for objects different than superweapons which are disableable from shell (for example heroes would be enabled when Tech Level is set to 10 while rest of the equipment is available at Tech Level 9).
I understand the sentiment of this request and there's nothing inherently wrong with giving modders the option of the good ol' tech level slider, but in all honesty, 3 or 4 extra game modes can be added to cover the majority of possibilities that a tech level slider would allow, and with the possibility to modify certain units to individually decrease the "tech level" of a unit (like a chrono miner with its teleport function disabled, or something like that).
Although the tech level slider is a bit more intuitive and perhaps a tad easier to understand, having "T1," T2," and "T3 game modes is a pretty simple and very flexible alternative with no drawbacks in functionality (as far as I am aware, at least).