Juho T. aka Squalius-cephalus, designer of the Silakkas, made a modular macro keyboard framework. The RP2040-based Suutari20 uses TRRS sockets and supports 3.5mm jacks from TS to TRRS.
I wanted a macro keyboard that I could quickly design and build modules for. The result is Suutari20, which uses TRRS connectors (don't worry, it won't short circuit like silakka54) and modules consisting of just a few components (switches and TRRS jack) – Squalius-cephalus.
The TRRS connector can be split into three mono connectors using a splitter. This allows you to connect three foot switches to one port for example.
Salim Benbouziyane shared the files of his Ocreeb MK2, a modular macropad system exploring the idea of a custom magnetic connector that lets you attach all sorts of external modules.