The map layer filter is part of Overthrow's map rework and is not in v1.4.
Overthrow's map draws a lot at once: every town, base, radio tower, FOB, port, camp, shop, gun dealer, warehouse, bus stop, house, vehicle, point of interest and waypoint it knows about, plus the territory shading and the restricted area rings underneath them all. The map filter is a panel that lets you decide which of those you want drawn.
The fullscreen map has a strip of tool icons down its left side. One of them is a filter, and selecting it opens the filter panel beside the strip. Selecting it again closes the panel.
The panel can be reached and used with a controller as well as with a mouse, and it is opened the same way either way. It needs no key or button of its own.
The panel is a list, and each row is both the legend entry and the switch for it.
Nothing is hardcoded into that list. If a later version of Overthrow adds a kind of marker or a new overlay, it turns up in the panel as another row.
Clearing a row hides that category straight away, with the map still open. Ticking it again brings it back the same way. Nothing has to be closed or reopened.
Everything starts switched on, so a player who never opens the panel sees exactly the map they would have seen anyway.
A category left switched on still keeps its own zoom threshold. Houses, shops and bus stops only draw once you are zoomed in far enough, and the filter does not change that. Switching a category on will not make it appear at a zoom level it was never drawn at.
The filter is a display choice and nothing more. It changes only what is drawn for you: it does not reveal anything the campaign was hiding, does not hide anything from the campaign's point of view, and has no effect on income, stability, spawns, patrols or AI. Other players see whatever they have chosen for themselves.
Your filter choices are saved against your game profile, not against a campaign save. They are waiting the next time you open the map, in a later session, and in any campaign or on any server you play in.
That also means starting a fresh campaign will not reset them. If the map looks emptier than you expect in a new campaign, the filter panel is the first place to look.