Is it true? Yes and no. It doesn't "decrease fuel pressure to the pump" as there is no pressure there, but rather a vacuum. What it does is make it harder for the pump to draw from the tank and the dirtier the filter gets the more it affects the draw. Fuel pumps are better at pushing than pulling so you don't want a restriction on the pull side The sock on the pickup in the tank is fairly coarse and is designed to keep the big chunks out of the pump. The filter between the pump and carb is designed to catch much smaller stuff that normally does not affect the pump but will affect the carb.
But will clarify, A paper filer before the pump may restrict the flow a small amount and technically require a bit more vacuum (which is negative pressure) But it is unlikely you would be able to measure the difference unless you were using laboratory grade instruments.
But as stated above if the filter starts getting dirty, the vacuum the pump produces will remain the same, but the flow will be reduced. The dirtier it becomes the lower the flow to the point it is completely blocked.
If you look at most fuel injection aftermarket systems they require a 10 micron filter Before the pump and a 3 micron after
