Toilet water line cloded
I'm beginning to wonder how many people only use the low-point drains in draining their systems instead of removing the Water heater anode to drain it first. If the anode is removed first (after opening faucets to relieve water pressure), all the white crud from the deteriorating anode comes out the port where the anode is installed. Low-point drains should be used last.
If the low-point drains are used first, some of the white crud comes out into the cold water line where it waits to be run into the toilet valve and bath faucets. See comment above by "Flybob".
The anode is the ultimate problem, not water filtration on the city water inlet side. The manufactured system, when new, is absolutely clean; no FOD or other particles. If a filter is needed, it is in the water delivery line immediately prior to the toilet valve.
Bottom line - check and change the W/H anode regularly.