When you unanchor a shipyard, display a radius for zero-decay production

So far in Base 2.3.3 (Huge) and 2.5 (Huge?) I haven't seen any decay at all, even from 150 hexes away.

Umm, there's quite a bit.  Click on your shipyard, click on Manage, click on Edit Sponsors.  There are 3 columns for Available Planets and Current Sponsors; the third with the red icon is 'Available Manufacturing' which shows how much production those planets can (for available sponsors) or do (for current sponsors) provide that shipyard.  Numbers in green have zero decay; numbers that are white have some (hover over the number to see how much exactly); numbers in red have a heavy decay.

If all your numbers are green and there's no decay, you must have a mod or have modified your XML files or something.

Nope.  No mod.  No decay.

For certain races perhaps.

Default United Earth race starts with decay. If however, you take the middle alignment path to get the engineering steps, all shipyard decay is removed with the second step, which is really nice.

Perhaps you have been doing the neutral alignment path?

It's Pragmatic, builder tree, Coordinated.

Yes, I'm Terran mostly Pragmatic.  Saw that circle didn't select it cuz I didn't need it.  I don't know what else to tell you.