Can a solo maintainer be considered to be an open-source software steward?

No. As defined in Article 3(14), an open-source software steward must be a legal person (e.g. a company, a foundation, an association) in contrast with a natural person (i.e. a human being). The obligations of open-source software stewards described in Article 24 therefore do not apply to solo maintainers acting in their personal capacity.

However, should a solo maintainer set up a single-member company that is a legal entity in its own right, distinct from the natural legal persona of the solo maintainer, that entity could qualify as an open-source software steward if it meets the criteria for doing so.

It is also worth noting that natural persons who monetise their project become subject to the CRA as manufacturers. In that case, they face the same obligations as any other manufacturer under Article 13, not the lighter-touch obligations that apply to open-source software stewards under Article 24. For more on what qualifies as monetisation, see Am I subject to the CRA if I earn a living from the open source project I maintain?.