Not just periods. Please allow all characters in passwords. Technically there is absolutely no reason to prevent any characters, just as there is no reason for an arbitrary limit of 20 characters. Both these problems go against current security best practices: use long passphrases consisting of multiple words, and/or use password managers to generate long strings of random characters (alphanumeric plus symbols). I'm a software engineer, I know how this works. PayPal, you're making yourselves look like amateurs.
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