In this case, since the lines of code that can touch the manually managed object pool are probably few and easily reviewed and audited, I don't have any problem

In this case, since the lines of code that can touch the manually managed object pool are probably few and easily reviewed and audited, I don't have any problem with your advice. I realize you're not advocating pervasive use of the technique, but if someone reading this is going to make pervasive use of manually managed object pools in a GC'd language, they should at least consider the possibility of moving to a language with both good language support for manually managed memory and a good ecosystem of tooling around manual memory management. Manually managed object pools in a language designed around GC don't fully get rid of the costs of GC, and re-expose the program to most of the errors (primarily use-after-free, double-free, and leaks related to poorly reasoned ownership) that motivated so much effort in developing garbage collectors in the first place.