A thundering, resonant NO, manual exercises do not lose thier effectiveness. While certain pressure can be attained with clamping, you can still grow with as little as jelqing, even after a long stint of clamping. In fact, the majority of all of the big gainers in Penis Enlargement history have done it by manual exercises alone. Jelqing still maintains its potency as a Penis Enlargement tool after learning squeezes and bends, so there is no reason why those manual exercises should be rendered useless.
In fact, after a while of clamping for me I started jelqing and doing other manual exercises again and got some of the best gains ever, not to mention my [words=https://shop.mattersofsize.com/products/sizegenetics-penis-extender]penis health[/words] improved drastically.
There is a concept of conditioning, where if you do the more advanced exercises the less advanced ones won't work as well because your penis has become "conditioned." I don't know if I buy into that theory, though the more gains you make, the more elusive the next ones become. But, if you are still making great gains (.1"+ a month) with manual exercises, don't expect them to double when you clamp. There is a limit to how fast your penis can be broken down and recover, and if you are at that threshold now, why try and speed it up to no avail?
Do not let it scare you from clamping. If you are truly concerned, why not make clamping just an addition for a while to see how you like it? Maybe jelq for 10 minutes, do a few squeezes, clamp for 5 minutes, etc.