Looking on the Fire-Lite website now, it appears that they have run out of the stock for the replacement MS-4 boards, but they are still selling/making(?) replacement MS-2 boards. Honeywell says that they will provide support until 2030.
The original discontinuation notice said that the MS-2 was discontinued immediately but the MS-4 had limited stock remaining. Do you have a link to where they say the MS-4 specifically is now out of stock? MS-2 & MS-4 Fire Alarm Control Panels|Honeywell Building Automation
For the MS-2 replacement boards, the only listed SKU is the DP-MS2/4, so that might also be discontinued.
As a side note, it looks like the W-SD355T has been discontinued but not the W-H355.