Note that limiting the maximum battery charge to something like 70 % would be more effective in those cases, completely avoiding the high voltages of a full charge.
However, even that seems absurd in such a use case. Does it matter if the old battery in an old phone gets worse? Especially when only used stationary anyway? Especially when the difference is sometime tiny like 5 % vs. 8 % (=3 % saved) capacity loss per year?