We have been discussing this at Multi-line tick labels · Issue #460 · observablehq/plot · GitHub but there is no official support yet. The idea would be to consider newlines in the tick format to, well, create a new line, as we do in Plot.text. Drafts are available at multiline ticks labels, draft by Fil · Pull Request #609 · observablehq/plot · GitHub and multiline tick labels by Fil · Pull Request #83 · d3/d3-axis · GitHub
(duplicated answer from d3.js - How to wrap long tick text in Observablehq using the Plot.plot function? - Stack Overflow)