There is no need for any sort of special license to publish 'compatible' adventures, supplements, etc. The only thing the OGL did was grant limited use to portions of the WotC trademarks and intellectual property. In other words, the OGL allowed 3rd party publishers to actually refer to specific, limited portions of actual D&D content.
All the current crop of 5e products are doing is publishing unique adventures that are 'compatible with the 5e of the most popular role playing game'. They aren't referencing any trademarked things (no mention of Dungeons & Dragons specifically, no Drow, etc) nor do they include any copywritten materials (no copy/paste of stat-blocks etc).
This has always been legal, and companies have been doing it as far back as 1st edition. The difference between this and publishing OGL based content back in the 3.0 era was that OGL content could actually refer to the SRD directly and make references to specific monsters and spell names that were included in the SRD, covered under the OGL (things like 'Drow' were still specifically trademarked and off-limits, however). For example, the SRD itself was released as open gaming content, and publishers could include it verbatim or copy/paste portions of it directly in their products if they wished. This included common races / classes / monsters and their stat blocks etc. However, WotC specifically left certain things out of the SRD / OGC, like the actual character advancement tables. This was an attempt to ensure that people had to purchase a Players Handbook to make use of the SRD and OGC. However, Paizo got around this with Pathfinder by making up their own character advancement tables.