Even if you found a "working" cracked 1.40.4.8 activation key, here's what you'd lose:

If you are looking for 1.40 specifically for mod compatibility (like older versions of ProMods), Steam allows you to "Opt-into Betas" and select previous versions of the game legally, provided you own the base license.

It completely overhauled the visual engine with realistic day/night cycles.

A "Product Key" is a unique, alphanumeric code that proves you have legally purchased a copy of software. When you buy a game like ETS2, you are essentially buying a license to use it. The product key is the digital token that validates that license.

Most keys generated by keygens are invalid, already used, or blacklisted by Steam, leading to a waste of time.