This works fine but needs some code improvements.
I modified your xml to fit

<?xml version="1.0" encoding="utf-8"?>
<extension version="3.2" type="plugin" group="content" method="upgrade">
	<name>plg_content_contact</name>
	<author>Joomla! Project</author>
	<creationDate>January 2014</creationDate>
	<copyright>Copyright (C) 2005 - 2017 Open Source Matters. All rights reserved.</copyright>
	<license>GNU General Public License version 2 or later; see LICENSE.txt</license>
	<authorEmail>admin@joomla.org</authorEmail>
	<authorUrl>www.joomla.org</authorUrl>
	<version>3.2.2</version>
	<description>PLG_CONTENT_CONTACT_XML_DESCRIPTION</description>
	<files>
		<filename plugin="contact">contact.php</filename>
	</files>
	<languages>
		<language tag="en-GB">en-GB.plg_content_contact.ini</language>
		<language tag="en-GB">en-GB.plg_content_contact.sys.ini</language>
	</languages>
	<config>
		<fields name="params">
			<fieldset name="basic">
				<field
					name="url"
					type="list"
					label="PLG_CONTENT_CONTACT_PARAM_URL_LABEL"
					description="PLG_CONTENT_CONTACT_PARAM_URL_DESCRIPTION"
					default="0"
				>
					<option value="0">PLG_CONTENT_CONTACT_PARAM_URL_0</option>
					<option value="1">PLG_CONTENT_CONTACT_PARAM_URL_1</option>
					<option value="2">PLG_CONTENT_CONTACT_PARAM_URL_2</option>
				</field>

				<field
					name="link_to_alias"
					type="radio"
					label="PLG_CONTENT_CONTACT_PARAM_ALIAS_LABEL"
					description="PLG_CONTENT_CONTACT_PARAM_ALIAS_DESCRIPTION"
					default="0"
					class="btn-group btn-group-yesno"
				>
					<option value="0">JNO</option>
					<option value="1">JYES</option>
				</field>
			</fieldset>
		</fields>
	</config>
</extension>

To get:

remark that we use JNO and JYES, therefore the strings

PLG_CONTENT_CONTACT_PARAM_YES="Yes"
PLG_CONTENT_CONTACT_PARAM_NO="No"

should be taken off the ini file.

Also the fields name do usually use lowercase characters.
I changed them to
url and link_to_alias
Which means they should be changed in code.

Also I would change
PLG_CONTENT_CONTACT_PARAM_URL_LABEL="URL to link to"
to maybe a simple
PLG_CONTENT_CONTACT_PARAM_URL_LABEL="Redirection"
as obviously an email is not an url :)

Also use lowercase for html tags in text strings, i.e. not <UL><LI> but <ul><li>

@infograf768 the > needs to be indented like this

					default="0"
					>
					<option value="0">PLG_CONTENT_CONTACT_PARAM_URL_0</option>

Should the version number also be upgraded?

I have another doubt: in the xml the "extension version" is set to 3.2, but in php code the "protected $db;" code says it's available since 3.3...

No need to touch the versions.

I have tested this item ✅ successfully on 5693292

Works fine here. May need some tweaks to the values of the ini strings. @brianteeman

I have tested this item ✅ successfully on 5693292

Handy Improvement, thanks.

RTC after two successful tests.

@mbabker Please decide a milestone for this.