The RedirectTo AMPscript function prevents link href values from getting recorded in tracking (C). When RedirectTo is used, it bypasses the standard link wrapping and tracking process, causing the link click not to be captured in the tracking data.
References:
Salesforce Marketing Cloud AMPscript Guide
Link Tracking in Marketing Cloud

To configure a data extension to store data for six months, you should update the data extension configuration to include data retention settings.
* Data Retention Settings: You can configure data retention directly within the data extension settings in Email Studio to specify how long data should be retained. This ensures that data older than the specified period is automatically deleted.
* Go to Email Studio -> Subscribers -> Data Extensions.
* Select the data extension you want to configure.
* Click on "Properties".
* Under "Data Retention", set the retention period to six months and define the retention settings accordingly.
Salesforce Data Extension Retention Settings

Data extension functions in AMPscript, such as LookupRows, LookupOrderedRows, and UpdateDE, can most negatively impact send processing because they involve querying and manipulating data extensions.
These operations can be resource-intensive and time-consuming, especially when dealing with large data sets.
Salesforce AMPscript Functions

To store only the first 4000 characters of the fields SMTPBounceReason and SMTPCode in a data extension, the developer should use the LEFT (C) SQL function. This function returns the specified number of characters from the beginning of a string.
Example:
SELECT SubscriberKey, LEFT(SMTPBounceReason, 4000) AS SMTPBounceReason, LEFT(SMTPCode,
4000) AS SMTPCode FROM _Bounce
References:
SQL LEFT Function
Salesforce Marketing Cloud SQL Reference

When an email is sent to a list where subscribers have no associated Subscriber Key, Marketing Cloud will use the Email Address as the Contact Key by default. This ensures that each subscriber can still be uniquely identified even if they do not have a predefined Subscriber Key.
Salesforce Subscriber Key Concepts