SharePointOnlineCredentials does not contain GetAuthenticationCookie

Question

I've accepted some work using SharePoint. I need to discover the APIs and call them. I've looked for examples and followed one that looks like this:

    var siteUrl = "https://contoso.sharepoint.com";
    var username = "user@contoso.onmicrosoft.com";
    var password = "pass@word1";
    var securePassword = new SecureString();
    password.ToCharArray().ToList().ForEach(c => securePassword.AppendChar(c));
    var credentials = new SharePointOnlineCredentials(username, password);

    var handler = new HttpClientHandler();
    handler.Credentials = credentials;

    var uri = new Uri(siteUrl);
    handler.CookieContainer.SetCookies(uri, credentials.GetAuthenticationCookie(uri));

    var json = string.Empty;
    using (var client = new HttpClient(handler))
    {
        client.DefaultRequestHeaders.Clear();
        client.DefaultRequestHeaders.Add("Accept", "application/json;odata=verbose");
        var response = await client.GetAsync(siteUrl + "/_api/Web");

        response.EnsureSuccessStatusCode();

        json = await response.Content.ReadAsStringAsync();
    }

I've got these libraries installed: sigh I pasted an image into SO and it does not work

Microsoft.Sharepoint.Client
Microsoft.Sharepoint.Client.Runtime
TICUE.NetCore.SharepointOnline.CSOM.16......

I note all doco says the first two would work and they did not. The last one gave me the class at all.

However, I still get an error that I can finally see the SharePointOnlineCredentials class, but the IDE insists that GetAuthenticationCookie is not a method. Pretty much every online instruction I've found did not work, does Sharepoint API just change constantly? Is there a real source of truth? The Microsoft docs I found say this method exists.

I created a DLL outside Maui in case it was a Maui issue but my core code is in a Maui project.