A board may be able to run a huge number of 8051 variants as long as they just make use of the classic 40-pin pinout.

But a board can normally not run all chips, since it is possible to have both 5V and 3,3V chips that makes use of the same pinout - but do the board support both voltages?

Next thing is that different 8051 chips may require different crystal frequency if you want to take advantage of the full speed of the chip. And in some situations, you may have specific 8051 chips that does not run well even with a crystal within the supported frequency range because they are a bit sensitive about crystal parameters and load capacitors.

Another thing is that not all 8051 supports ISP.

You can probably stumble on other issues too.

In the end, it is seldom a big need to support a large number of different manufacturers and chip families. Newer chips can normally match the functionality of older chips and it is way better to only do projects with ISP-capable chips. Staying within a single range of chips from a single manufacturer also means that they normally have similar extensions (extra UART, timers, I2C, ...) if you want to base different projects on chips with more or less capability/cost depending on needs.