Skills are a feature of the former Portfolio Live Plans, an older version of the tool that is no longer supported since July 31st (see this announcement).
While intrinsically a powerful feature, it took a ridiculous amount of work and commitment to actually use in real life scenarios: for every single issue on a roadmap, the feature required to split up estimates across all involved users and skills in order to generate a realistic roadmap. Atlassian has chosen a more pragmatic approach to planning in the new UI, which is also commented in the same announcement I linked above.
