From the doc of {strip}
I quoted in your other question (and asked you to read):
Anything within
{strip}{/strip}
tags are stripped of the extra spaces or carriage returns at the beginnings and ends of the lines before they are displayed. This way you can keep your templates readable, and not worry about extra white space causing problems.
One solution:
{strip}
{foreach name=foo from=$List item=Item}
{$Item.title}{if !$smarty.foreach.foo.last}{literal},
{/literal}{/if}
{/foreach}
{/strip}
Strip remove blanks and literal force ,<newline>
to be the separator. NB: spaces and newline are meaningful between {literal}
and {/literal}
(tested with smarty 3.1.16)