Take for instance a C150, below 5000' leaning is not permitted.
Not true.
I think you will find that leaning at FULL POWER below 5000' is not permitted.
Below 75% power, leaning is recommended at any altitude. Especially if you want to achieve published performance and fuel consumption, and avoid fouled spark plugs.
Of course it should not be at the expense of exceeding any other engine limitation.