This is a tricky one - none of us have done experiments to answer this question other than by informed guesses. If it goes down to half normal depth, I would guess no problem. When it starts to dry more than this, it starts to be a matter of preference, and perhaps.... https://twitter.com/scyrene/status/1274836358419316736

...a matter of your interests. A lot of drawdown has relatively little impact on water plants, though some species will not survive complete drying out (no sensitive submerged plants survive pollution). For adult amphibians its no problem. For newt, frog and toad tadpoles....

....complete drying is terminal, obviously enough, but they will survive in a half or even three quarters drawn down pond. Many pond animals are well adapted to live in very shallow, vegetation dominated water,and others can fly back in. Animals that like open water...

....- backswimmers, bigger diving beetles - probably won't like it, and will move on. If the pond is well shaped, the drawdown is a good thing, but it's hard to make garden ponds that exploit that feature. If you top up from the tap, you will save amphibian larvae from their....

....immediate demise (but remember amphibians have evolved to handle good and bad years). If you add the tapwater it's a trade-off: can you tolerate the pollution risks (fewer submerged plants, probably less life generally, a simpler community) vs saving particular reasonably...

.... tolerant creatures that can handle some pollution. These problems all go away if you can store rainwater - but this is actually the most challenging part of garden pond design (not the shape, depth, digging or planting) - but where do you get reasonably unpolluted water.