How would you implement automated testing that verifies deprecated API versions still function correctly while newer versions are actively developed?

A regression test suite organized by version (parameterized integration tests running the same test scenarios against each supported version's endpoint, using WebApplicationFactory with version headers/URLs varied per test case) ensures that ongoing development on newer versions doesn't accidentally break older, still-supported versions -- critical since deprecated doesn't mean 'can silently break', only 'scheduled for eventual removal', and consumers still relying on it deserve continued correctness guarantees until the announced removal date.

[Theory]
[InlineData("1.0")]
[InlineData("2.0")]
public async Task GetProduct_ReturnsCorrectShape_ForVersion(string version) {
    var client = _factory.CreateClient();
    var response = await client.GetAsync($"/api/v{version}/products/1");
    response.EnsureSuccessStatusCode();
    // assert version-appropriate response shape
}