While a Terms of Service (TOS) agreement is not a legal requirement, it helps your users understand your rules, requirements, and restrictions. It also helps you enforce these stipulations and maintain control over your website or app. There are plenty of examples and generators for terms of service text. You can require a user to agree with your TOS during registration.

You should store the date/time of consent in case you update the TOS and want your users to agree with the new terms. The Bootstrap Native Project (BSNP) implements ASP.NET Core Identity which supports user claims. I developed middleware which captures the current request path then redirects the user to a TOS page if the user doesn't have a TOS claim. The TOS page displays an agree button if the user's TOS claim is not found. When the user clicks the agree button, I add the TOS claim with the current date/time and redirect to the original path.

TermsOfServiceMiddleware.cs:

public class TermsOfServiceMiddleware
{
    private readonly RequestDelegate _next;
    public TermsOfServiceMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        if (context.User.Identity.IsAuthenticated &&
            context.Request.Path != new PathString("/identity/account/termsofservice") &&
            context.Request.Path != new PathString("/identity/account/logout") &&
            !((ClaimsIdentity)context.User.Identity).HasClaim(c => c.Type == ApplicationClaimType.TermsOfService))
        {
            var returnUrl = context.Request.Path.Value == "/" ? "" : "?returnUrl=" +
                HttpUtility.UrlEncode(context.Request.Path.Value);
            context.Response.Redirect("/identity/account/termsofservice" + returnUrl);
        }
        await _next(context).ConfigureAwait(true);
    }
}

public static class TermsOfServiceMiddlewareExtensions
{
    public static IApplicationBuilder UseTermsOfService(this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<TermsOfServiceMiddleware>();
    }
}

Notice the hint to scroll to the bottom for the agree button.

TermsOfService.cshtml.cs:

public string ReturnUrl { get; set; }
public bool ShowAgreeButton { get; set; }

public IActionResult OnGet(string returnUrl = null)
{
    returnUrl ??= Url.Content("~/");
    ReturnUrl = returnUrl;

    if(_signInManager.IsSignedIn(User) && !User.HasClaim(c => c.Type == ApplicationClaimType.TermsOfService))
        ShowAgreeButton = true;

    return Page();
}

public async Task<IActionResult> OnPostAsync(string returnUrl = null)
{
    if (!ModelState.IsValid) return Page();

    var user = await _userManager.GetUserAsync(User).ConfigureAwait(false);
    if (user == null) return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
    var claims = User.Claims.ToList();
    if (claims.Count == 0 || claims.Where(c => c.Type == ApplicationClaimType.TermsOfService).FirstOrDefault() == null)
    {
        var result = await _userManager.AddClaimAsync(user,
            new Claim(ApplicationClaimType.TermsOfService,
            string.Format("{0:MM/dd/yyyy}", DateTimeOffset.UtcNow))).ConfigureAwait(false);

        if (!result.Succeeded)
            throw new InvalidOperationException($"Error occurred setting TermsOfService claim " +
                $"({result}) for user with ID '{_userManager.GetUserId(User)}'.");

        await _signInManager.RefreshSignInAsync(user).ConfigureAwait(false);
    }

    returnUrl ??= Url.Content("~/");
    return LocalRedirect(returnUrl);
}