I wrote up the following to explain to myself why TIE/fos exist, and how they might be serving alongside older fighters on my table. I think it makes sense, but I'm interested in anyone else's opinion.
In the aftermath of the Battle of Endor, most fleets in the Imperial Navy found themselves with far fewer resources than they were accustomed. While TIE Fighters continued to be made at an impressive rate in factories still under the influence of the Moffs and Admirals that retained power over their individual fleets, the overall output was not up to pre-Endor standards. More importantly, as many worlds declared for the Rebellion, the Fleets could not expect the same quantity and quality of recruits to the academies. The remnant Fleets that survived in the years after the battle of Endor found that their tactics for the employment of their TIE Fighter squadrons had to change. Their experienced pilots became far more valuable, and needed to be protected. In part, this was accomplished with tactics. Improved capital ship/starfighter coordination, less aggressive fighter tactics, and judicious use of reserves substantially reduced casualties in front line TIE/ln squadrons.
In addition, field modifications to stock TIE/lns began appearing. In earlier years, a particular promising pilot may have been transferred to a higher prestige unit equipped with more capable TIE variants. With a breakdown in supply, and a breakdown in cooperation between the higher level Imperial commanders and their fleets, the TIE squadrons became more static, personnel became citizens of their particular Fleet as much as citizens of the Empire, and the flow of equipment and personnel drastically slowed. Often, the only TIE variant that fleets could reliably source replacement parts for was the standard TIE/ln. In order to improve their capabilities and protect their increasingly more valuable pilots, many squadrons were allowed to add modifications to these fighters. Additional armor plating or lightweight shield generators were sometimes added. Improved avionics and engines, stripped from advanced fighters that could no longer fly or purchased from local sources, were sometimes fitted as well. Over the years, the modifications that were proven most effective began to be produced as refit kits in allied factories, and eventually TIE Fighters were produced with the modifications factory standard. These were the progenitors of TIE Fighters used by the First Order decades later.