What percentage of software engineers leave the Information Technology
sector entirely within a decade or two of entering it? (Someone who
was doing something that still required the software knowledge, for
instance, managing software engineers, or being an intellectual
property lawyer, would still be "in the IT sector", but someone who
became a civil engineer or a doctor probably wouldn't.)

Howdy gremlin,

This might get you (or someone else) a start on your question.

If you do not have it, you will need the free Adobe Acrobat Reader
from:
http://www.adobe.com/products/acrobat/readstep2.html

Here is the total and net replacement rates for software engineers,
etc.
from the U.S. Department of Labor Bureau of Labor Statistics.

www.bls.gov/emp/emp_repl.pdf - Page 10

Table 11. Total and net replacement rates and annual average
replacement needs, 2000-10 — (Numbers in thousands)

The columns of numbers are titled in order:

Total employment 2000
Replacement rate (Percent) - Total, 1999-2000
Replacement rate (Percent) - Net, 2000-2010 - Annual average
Replacement rate (Percent) - Net, 2000-2010 - 10-year
Annual average, replacement needs, 2000-10 - Total
Annual average, replacement needs, 2000-10 - Net

Computer and mathematical occupations ........... 2,993 7.4 1.0  9.7 
221 29
Computer specialists ............................ 2,903 7.3 0.9  9.2 
212 27
Computer programmers .............................. 585 6.1 2.1 21.0  
36 12
Computer scientists and systems analysts .......... 459 7.9 0.9  8.8  
36  4
Computer and information scientists, research ...... 28 7.9 0.9  8.8  
 2  -
Computer systems analysts ......................... 431 7.9 0.9  8.8  
34  4
Computer software engineers ....................... 697 7.4 0.7  6.7  
51  5
Computer software engineers, applications ......... 380 7.4 0.7  6.7  
28  3
Computer software engineers, systems software ..... 317 7.4 0.7  6.7  
23  2
Computer support specialists ...................... 506 7.9 0.4  4.2  
40  2
Database administrators ........................... 106 7.9 0.4  4.2  
 8  -
Network and computer systems administrators ....... 229 7.9 0.4  4.2  
18  1
Network systems and data communications analysts .. 119 7.9 0.4  4.2  
 9  1
All other computer specialists .................... 203 7.9 0.9  8.8  
16  2

You should go to the web file for details, and more readable chart.


Search strategy

"software engineers" "change careers" percentage
://www.google.com/search?q=%22software+engineers%22+%22change+careers%22+percentage

Looking Forward, denco-ga