versioninfo()

using DataFramesMeta

panel_data = DataFrame(
    sample_id = [1, 1, 2, 2, 3, 4, 5, 5],
    treatment = ["T", "T", "F", "F", "F", "T", "F", "T"],
    measure   = [2, 3, 1, 1, 4, 3, 4, 5]
)

function latest(treatment, measure)
    i = findlast(==("T"), treatment)
    isnothing(i) ? missing : measure[i]
end

@linq panel_data |>
    groupby(:sample_id) |>
    combine(
        has_treated = any(==("T"), :treatment),
        initial_value = first(:measure),
        latest_value_when_treated = latest(:treatment, :measure)
    )